1. Field of the Invention
The present invention relates to a charge pump. More particularly, the present invention relates to a low charge-injection charge pump for use in low noise, low power, and high speed networking or telecommunications systems.
2. Discussion of the Related Art
Charge pumps are an important element of many electronic circuits such as phase locked loop (PLL) circuits. For example, FIG. 1 illustrates a typical PLL circuit having a phase-frequency detector 11 and a conventional charge pump and loop filter 21 driving a voltage controlled oscillator (VCO) 31. When the phase-frequency detector 11 detects that the phase and frequency of a reference clock CKr is greater than the phase and frequency of the feedback clock CK1, an UP signal is generated. The UP signal causes the charge pump and loop filter 21 to incrementally increase the output voltage VC1 thereby increasing the frequency of feedback clock CK1. When the phase-frequency detector 11 detects that the phase and frequency of the feedback clock CK1 is greater than the phase and frequency of the reference clock CKr, a DN signal is generated. The DN signal causes the charge pump and loop filter 21 to incrementally decrease its output voltage VC1 thereby decreasing the frequency of feedback clock CK1. When the reference clock CKr and the feedback clock CK1 are synchronized, neither the DN signal nor the UP signal is received by the charge pump and loop filter 21 and the voltage VC1 remains constant.
The structure and operation of a conventional charge pump will now be described. Typically, a charge pump generates an output voltage on a charge summing node by dumping charge onto the node from a current source via a switch, or by sinking charge from the charge summing node to a current sink via a switch. An output voltage VC1 is generated by the accumulated charge present on the charge summing node.
FIG. 2 shows a conventional charge pump circuit employed by the prior art PLL of FIG. 1. The conventional charge pump circuit includes a current source 22, a current sink 26, an UP switch 23, a DN switch 25, and an RC loop filter 28. The current source 22 is connected between VDD and the UP switch 23 while the current sink 26 is connected between the DN switch 25 and ground. When the UP signal is asserted, the UP switch 23 closes allowing current to flow into the charge summing node, thereby causing the voltage VC1 to increase. When the DN signal is asserted, the DN switch closes allowing current to flow out of the charge summing node 24, thereby causing the voltage VC1 to decrease. An RC loop filter 28 is connected to the output of the charge pump to reduce the speed at which VC1 varies.
During the time when the UP switch 23 (e.g., a PMOS switch) is turned ON (closed), a channel charge exists in a channel located between the source and drain of the switch 23. When the switch is turned OFF (opened), the channel collapses causing a portion Qc1 of the channel charge to flow into the current source 22 and causing a portion Qc2 of the channel charge to flow into the charge summing node 24. Likewise, during the time when the DN switch 25 (e.g., an NMOS switch) is turned ON, a channel charge exists in the channel between the source and drain of DN switch 25. When switch 25 is turned off, the channel collapses causing a portion Qc4 of the channel charge to flow into the current source 26 and causing a portion Qc3 of the channel charge to flow into the charge summing node 24. These charges flowing into the charge summing node 24 introduce undesirable transients on the line between the charge summing node and VC1 thereby causing the voltage VC1 to fluctuate. In a case where the output VC1 of the charge pump drives the control input of a VCO in a PLL, these fluctuations can introduce undesirable jitter in the output clock of the PLL.
A need therefore exists for an improved charge pump that solves the problems described herein by reducing the effect of the voltage fluctuations caused by charge injected electrons and holes disturbing the charge summing node when charge pump switches are turned off.